1. Field of the Invention
This invention relates to a method and apparatus in which high-intensity ultrasonic vibration is applied to a radiator that creates cavitation bubbles, breaks up purge gas (e.g., argon or nitrogen) which is intentionally introduced in a small amount into the melt in order to collect the cavitation bubbles and to make the cavitation bubbles survive in the melt, and induces acoustic streaming to disperse the bubbles uniformly in the melt, resulting in a fast and clean degassing of the molten metal or molten metal alloy.
2. Description of the Related Art
Porosity is one of the major defects in the casting of aluminum alloys, magnesium alloys, and steels. The formation of porosity is due to a significant solubility difference of gaseous elements in the liquid and solid phases, and an inadequate feeding of the solidification shrinkage of the alloy. For example, in aluminum alloys, hydrogen solubility in molten alloy is much higher than that in the solid. As a result, hydrogen porosity forms during the solidification of aluminum castings. The presence of porosity can be detrimental to the mechanical properties and corrosion resistance of the castings. Molten aluminum and aluminum alloys also contain solid non-metallic inclusions (e.g. aluminum oxides, aluminum carbides) and various reactive elements (e.g. alkali and alkaline earth metals). Non-metallic solid inclusions reduce metal cleanliness and the reactive elements and inclusions may create unwanted metal properties.
Therefore, before many molten metals and alloys are used for casting, unwanted components that may adversely affect porosity or other physical or chemical properties of the resulting cast product are removed. These unwanted components are normally removed from molten metals by introducing a purge gas below the surface of the molten metal. As the resulting purge gas bubbles rise through the mass of molten metal, the gas bubbles adsorb gases (e.g., hydrogen) dissolved in the metal and remove them from the melt. In addition, non-metallic solid particles are swept to the surface by a flotation effect created by the bubbles and can be skimmed off. If the gas is reactive with contained metallic impurities, the elements may be converted to compounds by chemical reaction and removed from the melt in the same way as the solid particles. This process is often referred to as “metal degassing”, i.e. reducing the gas content of a molten metal or metal alloy.
Several methods have been proposed for degassing molten metals and molten metal alloys. For example, U.S. Pat. No. 6,887,424 describes a process for inline degassing of a molten metal using a rotary device for generating bubbles of inert gas in the molten metal. U.S. Pat. Nos. 5,660,614 and 5,340,379 describe devices for injecting gas into molten aluminum. However, in these devices, degassing is a relatively slow process due to the large size of the bubbles that are produced. Other problems associated with these methods are the failure of moving graphite parts and the disturbance of the molten melt surface during rotary degassing, resulting in a significant oxide formation. Furthermore, the purging gas (usually argon) has to contain a few percent of chlorine in order make degassing efficient. Chlorine may be detrimental to the environment.
Molten metal degassing devices using ultrasonics have also been proposed. For example, Japanese patent application JP 2-173205 describes a dipping pipe of a degassing machine having an ultrasonic vibrator attached to the inner wall of the dipping pipe through which molten metal is drawn from a ladle into a vacuum tank. Japanese patent application JP 2-173204 describes a vacuum tank of a degassing machine having an ultrasonic vibrator attached to the bottom wall of the vacuum tank including dipping pipes for circulating molten metal. These methods utilize ultrasonics to assist vacuum degassing but vacuum degassing is seldom used in North America.
High-intensity ultrasonic vibration has been used to create cavitation bubbles for degassing molten metals. These methods can be used for degassing melts of small volume because hydrogen diffused to the cavitation bubble can diffuse back to the molten metal and a part of the cavitation bubbles can not survive for a longer time.
There is still a need for an improved apparatus and method for the degassing of molten metals or molten metal alloys and to eliminate the use of chlorine in the purging gas.